=AVERAGE(AVERAGEIFS(Completes!$E$2:$E$1000,Complet es!$D$2:$D$1000,{"12/1/2008","11/1/2008","10/1/2008"}))
=median?

I'm need to do an average & median of the 3 month total of defects per
completed product from a factory. The formula above seems to come close for
the average I need but it is averaging each month seperate and then averaging
those 3 totals instead of averaging all 3 months as 1 group of numbers.

On the Completes tab each row has the part number, serial number, month
completed, defects total for that serial number.

SERIAL_NO START_DATE COMPLETE_DATE Month Defects
3100 29-Jan-09 29-Jan-09 Jan-09 1
36189 21-Jan-09 21-Jan-09 Jan-09 3
36190 05-Jan-09 05-Jan-09 Jan-09 6
Please help :)